One of the difficulties in conventional systems for treating contaminated groundwater arises from the fact that taking water out of the ground for treatment is expensive. From the cost point of view, it is highly desirable that the groundwater be treated in-situ, ie while remaining in the ground.
It has been proposed to place treatment material in the ground, the material being inserted into a trench which has been excavated in the path of a plume of contaminant. The nature of the treatment material of course depends on the nature of the contaminant. An example is described in WO-91/08176, published 13 Jun. 91, in which the contaminant is an halogenated hydrocarbon, and the treatment material is iron. The iron is in the form of iron filings, which are placed in a trench excavated down into the material of the aquifer in the path of the velocity of the halogenated hydrocarbon contaminant plume.
When groundwater water is treated in-stiu or in-ground, many of the expenses of above-ground treatment, such as the expense of disposing of the treated water, can be avoided. Inexpensive though in-ground treatement usually is, however, still, in practice, it can be difficult and expensive actually to provide a trench of the required depth and extent in the material of the aquifer.
Aquifer materials often include large proportions of sand and gravel. One of the difficulties in providing a trench lies in the fact that such materials have little structural cohesion. In fact, sand/gravel material is especially unable to support itself when saturated with groundwater, ie below the water table, which is where an in-situ treatment system is particularly required to operate. It may be regarded that saturated sand/gravel aquifer material, from the standpoint of excavating a trench, has hardly any more ability to support itself than if it were a liquid.
One conventional manner for providing trenches in an aquifer material is to insert a pair of barriers, made of sheet metal, and inserted by pile-driving, down into the material of the aquifer. The two barriers are disposed in a spaced, parallel relationship. After the barriers are in place, the aquifer material is then extracted from between the barriers. Treatment material (eg iron filings) is inserted between the barriers, and replaces the sand/gravel that has been extracted. The two barriers are removed once the treatment material is in place.
The treatment material in the trench thus acts as a curtain through which the contaminated groundwater passes. The contaminant undergoes treatment--preferably chemical or microbiological breakdown--in passing through the treatment material. As such, of course the treatment material must be permeable.
The velocity with which groundwater travels in an aquifer of course varies a great deal from place to place, and from time to time, but velocities of the order of a few cm, or a few tens of cm, per day are typical. If the trench is say 1 meter wide, the groundwater has a residence time in the trench of perhaps a few hours or a few days. Generally, this is ample time for the chemical breakdown of the contaminant to occur, and for the breakdown reactions to be fully completed.
However, many treatment materials are themselves expensive. It can be desirable therefore to mix the treatment material with a less expensive material, and preferably a material which is chemically inert or passive with respect to the contaminant breakdown reaction. One material that might be used, for instance, is the sand/gravel that has been taken out of the trench.
It should be noted that it is difficult to make a trench of less than about a meter in width; on the other hand, however, even if it were possible practically to construct narrower trenches, a wide trench is preferred in order that the residence time be kept high.
In summary, one of the conventional ways of preparing a trench is this: pile-drive two parallel barriers; extract the material from between the barriers; form a mixture of the treatment material with the just-extracted material; insert this mixture into the space between the barriers; and then remove the barriers from the ground.
Although cheap compared with taking the water out of the ground, this procedure can still be expensive. The procedure must also be regarded as inefficient: even if the treatment material comprises only say 5% of the mixture inserted into the trench, the whole of the volume of the trench nevertheless had to be excavated. The inefficiency arises because much, or most, of the sand/gravel extracted from the trench is put straight back.
The present invention is aimed at providing a system whereby the plume of contaminated groundwater is caused to pass through a curtain of treatment material, but the manner of placing the treatment material in the aquifer, in the path of the plume, is carried out much more economically and efficiently than has been the case with the conventional systems.
The invention is aimed at providing a system whereby the plume of contaminated groundwater is caused to pass through a curtain of treatment material, but the manner of placing the treatment material in the aquifer, in the path of the plume, is carried out much more economically and efficiently than has been the case with the conventional systems.
The invention is aimed at providing a system for in-ground treatment of contaminated groundwater. As in the "trench" system, the contaminated groundwater is passed through a body of treatment material. The system of the invention provides a watertight wall, with gates. Receptacles for the treatment material are provided in the gates. The watertight barrier may be formed from sealable pile-driven elements, and the receptacle formed from an enclosure made from pile-driven elements and linked by waterproof junctions to the barrier.
The barrier should be watertight, apart from the gates: that is to say, no water should be allowed to emerge through the barrier unless that water has passed through the treatment material.
(In this specification, the term "watertight" is intended to be construed in the sense of "substantially watertight". That is to say, insofar as a particular barrier may permit some leakage, that the barrier is watertight, as that term is used herein, if the amount of the leakage is negligible in the context of the operation of cleaning up the contaminted groundwater. If the nature of the pollutant is such that absolutely no trace of the pollutant can be permitted to escape, then the barrier must be absolutely watertight. If, as is more usual, the pollutant can in fact be permitted to be present up to some threshold concentration, then the barrier is watertight, as that term is used herein, if the barrier permits only so little leakage as to enable that threshold requirement to be met.)
There are other types of walls which may be classed as watertight, and the invention may be used in connection with such walls. Examples of watertight in-ground wall structures include: interlocking plastic sheet walls; concrete column walls; soil-bentonite slurry walls and other types of impervious slurry walls; concrete panel walls; vibrated beam bentonite or cement walls; auger mix walls; jet grout walls; and the like.
An aim of the treatment system of the invention is to provide a system in which different in-ground components of the treatment system can be joined together quickly, cheaply, and reliably. A particular aim of the invention is to provide a system in which a watertight wall is joined to a gate, and the junction between them is also watertight.
It is an aim of the invention also to provide a treatment system in which only a single type of (heavy) technology is required for its installation. It is recognised that it is desirable to avoid a system in which, for example, heavy excavation equipment had to be provided at the site, and in addition, heavy concrete insertion machinery was also needed for other phases of installation. It is an aim of the invention to provide a treatment system in which, where the watertight wall is of sealable pile-driven elements, the receptacles can be installed wholly by pile-driving. This is not to say that other technologies are ruled out in the invention; but such other activities as extracting spoil from an enclosed receptacle, for example, require only light equipment to be brought to the site, as compared with the equipment needed, for example, to excavate a trench.
The invention makes use of watertight barriers. It is conventional for such barriers to be made of sheet steel elements, and to be pile-driven into the ground. Watertight barriers are also known, in which a separate massive structure is first hammered into the ground, and then a comparatively light sheet is inserted into the cavity created thereby. It is also known for the sheets of in-ground barriers to be made of plastic.
The invention is concerned with a waterproof barrier provided with gates in which are placed receptacles of treatment material, and with the manner by which such gates are joined in a watertight fashion to the watertight wall. As such, the invention is applicable to other watertight barrier systems, apart from the (preferred) waterproof sheet-piling barriers.